In many hydraulically actuated systems, such as hydraulically actuated fuel injectors, cryogenic pumps and the like, two-position, three-way spool valve assemblies control the operation of hydraulic components of the system. In general, the spool valve assembly opens an internal work fluid passage to a high pressure supply passage when the hydraulic fluid is supplied to the hydraulic component, such as to hydraulically move a piston. Between operation events, the internal work fluid passage is opened to a low pressure fluid passage or drain so that the used pressurized fluid in the internal work fluid passage can be removed, and the hydraulic component reset for its next operation event. In some cases, the operation of the spool valve assembly is controlled by a pilot valve member that is moved by a solenoid actuator to alternately provide and cut off pilot fluid that causes a valve spool to move between positions to control the flow of hydraulic fluid to and from the internal work fluid passage.
One example of a spool valve assembly is disclosed in U.S. Pat. No. 6,283,441 issued on Sep. 4, 2001, to Tian and entitled, “Pilot Actuator and Spool Valve Assembly.” The Tian patent disclosed a pilot actuated spool valve assembly that includes a valve body that defines a first passage, a second passage and a flow passage. A unitary pilot body is at least partially positioned in the valve body and has a first valve seat and a second valve seat. A pilot member is moveable in the pilot body between an up position in which the pilot member closes the first valve seat and a down position in which the pilot member closes the second valve seat. A spool valve member has a control hydraulic surface and is positioned in the valve body. It is moveable between a first position in which the first passage is open to the flow passage, and a second position in which the second passage is open to the flow passage. At least one of the valve body, the pilot body, the pilot member and the spool valve member define a control volume fluidly connected to the first passage when the pilot member is in one of its up position and its down position. The control hydraulic surface of the spool valve member is exposed to fluid pressure in the control volume. The disclosed valve assembly finds application in a hydraulically actuated fuel injector having a direct control needle valve.